388 Mr. 0. Fisher on the Thermal Conditions and 



bottom) and the most recent (which will be at the top) must 

 have its corresponding layer. 



Joining the corresponding layers in contiguous columns, it 

 appears that there must be a regular stratification, as was ob- 

 served to be the case, and that the strata will not deviate far 

 from horizontally. The question then arises, Where were 

 these strata deposited ? and what regulates their thickness ? 



Fig.l. 



Let a cylindrical surface A B be described in the ice-cap 

 around the polar axis P 0, with radius r approximately equal 

 to P A. Also describe a ring on the surface of the ice, whose 

 width is w n ; and Jet p n be the distance of the edge of it further 

 from the pole. NMisa section of this ring, and PN=p B ; 



Now, if the form of the ice-cap is permanent, the snow which 

 falls upon the ring, and upon any part of it, will always follow 

 the same route to meet the cylindrical surface at A B. Sup- 

 pose NK to be the route taken by the particles which travel 

 from N, and M L by those from M. It is evident, then, that 

 the ice at A B will consist of layers, each of which is continu- 

 ally fed by the snowfall upon its own corresponding ring- 

 fixed in position upon the surface, and that the rings nearer 

 the pole will supply the lower layers. This conclusion is in- 

 dependent of any assumption as to the forms of the paths 

 pursued by the ice. It only supposes that these paths do not 

 cross one another. 



Suppose that AB = /i, KL = S/i n . Then, if s be the depth of 

 the annual snowfall upon N M, we must have the quantity of 

 ice which passes outwards each year through the cylindrical 

 surface at K L equal to the annual snowfall on the ring at N M. 



Now the area of the ring at N M is 



Taking t> to represent the mean annual velocity through K L 

 per annum, we must have, from the above consideration, 



